R-Value Calculator
This R value calculator finds the thermal resistance of insulation from its thickness and R-value per inch, or converts a U-value to an R-value. Enter the values and the R value calculator returns the total R-value, the metric RSI, and the U-factor. It works as an insulation R value calculator and an R value to U value converter in one tool.
Results
The R-Value Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the R value calculator shows
R-value measures how well a layer resists heat flow; the higher the number, the better it insulates. The R value calculator multiplies insulation thickness by its R-value per inch and adds any other layers. It also works as an r value converter, giving the metric RSI and the U-factor, which is the inverse of R.
R value to U value formulas
R-value adds through layers, and U-value is its inverse:
R here is the imperial unit (ft²·°F·h/BTU). RSI is the metric unit (m²·K/W). To go from U value to R value, divide 1 by the U-value.
R value calculator wall example
A 3.5 inch wall cavity filled with batts rated R-3.7 per inch. The R value calculator wall result is:
Enter a U-value of 0.05 instead and the u value to r value calculator mode returns R-20.
How to use the insulation R value calculator
Enter the insulation thickness in inches, its R-value per inch, and the R-value of any other layers such as sheathing or air films. The insulation R value calculator returns the total R-value, RSI, and U-factor. If you have a U-value from a product sheet instead, enter it in the U-value box and the tool converts it straight to R.
Reading the r value converter result
Compare the total R-value with your local energy code for walls, roofs, and floors. U-factor is used for windows and whole assemblies, where lower is better. Studs, joists, and gaps conduct heat around the insulation, so a whole-wall R-value is lower than the cavity R-value this tool reports for the insulation alone.
When to use the attic insulation R value calculator
- Checking attic depth with the attic insulation R value calculator
- Comparing batts, blown-in, and foam board
- Converting R value to U value for energy reports
- Working out how much to add to existing insulation
Common R value calculator mistakes
- Ignoring framing. Studs lower the whole-wall R-value.
- Mixing R and RSI. Imperial and metric values differ by a factor of about 5.7.
- Compressing batts. Squashed batts lose R-value.
Tips for accurate results
- Use the manufacturer's R per inch, not a generic value.
- Seal air leaks before adding insulation.
- Allow for settling with blown-in material.
Calculate R value of wall assemblies
To calculate R value of wall assemblies, add the R-value of every layer in series: outside air film, cladding, sheathing, insulation, drywall, and inside air film. That sum gives the clear-wall R-value. Framing members have a lower R-value than insulation, so the true whole-wall figure is a weighted average. Better insulation lowers the heating and cooling load, which the BTU calculator turns into equipment size. For insulation sold by thermal resistance, this is the r value for insulation you compare across products.
Attic R value calculator for added layers
When topping up an attic, measure the existing depth, multiply by its R per inch, and add the new layer. An attic r value calculator shows whether the total meets code. Blown-in insulation settles over time, so add a little extra depth at installation.
Insulation R value calculator reference
| Material | R per inch | 3.5 in | 6 in |
|---|---|---|---|
| Fibreglass batt | 3.1 to 4.3 | R-11 to 15 | R-19 to 26 |
| Blown cellulose | 3.2 to 3.8 | R-11 to 13 | R-19 to 23 |
| Mineral wool | 3.0 to 4.2 | R-11 to 15 | R-18 to 25 |
| Closed-cell foam | 6.0 to 7.0 | R-21 to 25 | R-36 to 42 |
R-Value Calculator FAQ
How do you find the R value of insulation?
Multiply the thickness in inches by the R-value per inch, then add other layers. The R value calculator does this for you.
How do I convert R value to U value?
U = 1 ÷ R. An R-20 assembly has a U-value of 0.05.
What is RSI?
The metric R-value in m²·K/W. Multiply imperial R by 0.1761 to get RSI.
What R-value do I need for an attic?
It depends on climate; many codes ask for R-38 to R-60 in attics. Check your local energy code.